Chemical ionization at atmospheric pressure (APCI=Atmospheric Pressure Chemical Ionization) is a known method for creating analyte ions in ion mobility spectrometers (IMS) and mass spectrometers. In this context, atmospheric pressure refers to a pressure of between 6×104 and 1.2×105 Pascal.
In chemical ionization, the molecules of a gas containing analyte molecules, referred to below as the carrier gas, are first ionized by interaction with nuclear radiation (α, β, or γ radiation), or with electrons, X-ray quanta, UV light or in combination. In a cascade of primary reactions, the so-called reactant ions are created. The analyte molecules are ionized by secondary reactions with the reactant ions. These secondary reactions include the transfer of electrons, protons and other charged species from the reactant ions to the analyte molecules. Negative or positive analyte ions are created, depending on the properties of analyte and reactant molecules.
APCI ion sources are employed in mass spectrometry, in particular in combination with chromatographic separation processes such as gas chromatography (GC/MS) and liquid chromatography (LC/MS).
IMS devices with drift gas at atmospheric pressure are primarily employed for the detection of trace organic vapors from drugs, pollutants, warfare agents and explosives in the air and on surfaces. Apart from the most commonly used time-of-drift type (see FIG. 3), there are other less widely employed ion mobility spectrometers, such as the “Differential Mobility Spectrometer” (disclosed by Miller et al. in U.S. Pat. No. 7,005,632 B2), the “Field Asymmetric Waveform IMS” (disclosed by Guevremont et al. in U.S. Pat. No. 6,806,466 B2) or the “Aspiration Type IMS” from the Finnish company Environics Oy (disclosed in patent publication WO 94/16320 A1).
In almost all atmospheric pressure IMS systems used commercially, the analyte ions are generated by radioactive APCI ion sources; beta emitters such as tritium (3H) and, in particular, 63Ni are used, and also the alpha emitter 241Am. The mean kinetic energy of the electrons from beta emitters is between about 5 and 16 keV. Due to the restrictions that surround the use of radioactive sources, non-radioactive ionization methods have also been investigated since work began on IMS. This work has concentrated on photoionization and on corona discharge. Experience has shown that both methods involve different ionization processes from those of a 63Ni source, leading to different types of analyte ions: see, Dzidic et al.: “Comparison of Positive Ions Formed in Nickel-63 and Corona Discharge Ion Sources Using Nitrogen, Argon, Isobutene, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization-MS”, in: Anal. Chem., 1976, Vol. 48, No. (12), pages 1763-1768. Some analyte molecules cannot be ionized at all by these methods, which therefore do not represent equally effective alternatives to the radioactive electron sources 63Ni, 3H and the alpha emitter 241Am.
The patent specifications DE 196 27 620 C1 and DE 196 27 621 C2 from Budovich et al. disclose non-radioactive APCI ion sources in which electrons are generated in a vacuum chamber using a non-radioactive electron source and reach an electron capture detector (ECD) or the reaction region of an IMS by passing through a window that is permeable to electrons but impermeable to gas. The primary and secondary reactions of the chemical ionization take place in the ECD chamber or in the reaction chamber of the IMS after the electrons have entered through the window. In one embodiment, the window has a plane disk of mica between three and five micrometers thick which, having a diameter of five millimeters, is able to withstand a pressure difference of one atmosphere. It has, however, been found that the ion currents in IMS are significantly smaller than when a commercial 63Ni source with an activity of 100 MBq, corresponding to the currently permitted limit, is used. The result is a worse signal-to-noise ratio and a markedly poorer detection limit for analytes. The reason for this is that the mica disk is not sufficiently permeable to electrons with an energy of around 15 keV.
Electron sources with windows that are impermeable to gas but permeable to electrons are known from other applications, such as Ulrich et al. (“Anregung dichter Gase mit niederenergetischen Elektronenstrahlen” (“Excitation of Dense Gases with Low-Energy Electron Beams”), in: Physikalische Blätter, 56 (2000), No. 6, pages 49-52), and show that windows with a plane membrane of silicon nitride, having a thickness of only 200 to 300 nanometers, can be produced. A window of this type can withstand a pressure difference of one atmosphere if the surface area of the thin silicon nitride membrane does not exceed one square millimeter. A further electron source is described by F. Haase et al. (“Electron permeable membranes for MEMS electron sources”, in: Sensors and Actuators A: Physical, Vol. 132 (2006), No. 1, pages 98-103). In this, a plane membrane of silicon nitride, only 100 nanometers thick is mounted on a supporting honeycomb of silicon. The supporting structure is between 5 and 10 micrometers thick. The diameter of each honeycomb cell is around 10 micrometers.
A proportion of the electron energy always remains in the window when an electron passes through. The main effect of the absorbed energy is to heat the window up, but secondary electrons and X-ray quanta are also generated. The electron-permeable region of the window will be referred to below as the “window membrane”. The supporting frame for the window membrane (“window frame”) is significantly thicker than the window membrane, and therefore exhibits greater thermal conductivity than the membrane. In thermal equilibrium there is a temperature gradient between the hotter center of the window membrane and its supporting frame. The inhomogeneous temperature distribution generates mechanical stresses in the window, which the window membrane must be able to withstand. Furthermore, the heating-up of the window has the effect that the window membrane becomes more permeable to gas, and consequently the pressure in the vacuum chamber can rise to a point where the function of the electron source is no longer assured.